There are a number of methods for judging the functioning of a catalytic converter. One known approach for judging the functioning of a catalytic converter is based on the principle of amplitude evaluation, while another known approach is based on frequency evaluation.
A method using frequency evaluation is known from German Published Patent Application No. DE 34 43 649. In accordance with the method described therein, a two-position mixture control for the engine is effected. The mixture control is namely effected with the help of the signal of a probe arranged behind the catalytic converter as an actual control signal. In accordance with this method, the control frequency of the mixture control is determined and compared with a frequency threshold value. If the control frequency is found to be higher than the frequency threshold value, the catalytic converter is considered to be in condition for replacement. This method is based on recognition of the fact that if a catalytic converter is converting well, it takes a relatively long time until a change in the mixture, making the mixture richer or leaner, affects the exhaust flow leaving the catalytic converter. This long dead time after a change in the mixture from rich to lean, or from lean to rich, leads to a lower control frequency. The poorer the catalytic converter converts, the shorter the dead time and therefore the greater the control frequency.
A method for judging the functioning of a catalytic converter based on amplitude evaluation is described in U.S. Pat. No. 4,622,809. In the method described therein, a two-position mixture control for an engine is effected using the signal of a probe arranged in front of the catalytic converter as a control signal. By means of a probe arranged behind the catalytic converter, it is determined whether the control variations of the oxygen content which occur at the inlet of the catalytic converter are also noticeable at the outlet of the converter. As long as the catalytic converter is converting well, no control oscillation can be noted at its outlet. With aging of the catalytic converter, however, oscillations of the oxygen content of the exhaust gas also occur at the probe behind the catalytic converter. Such oscillations in oxygen content can also be intentionally produced by operating the engine with changed control parameters which lead to greater amplitudes of oscillation at the inlet of the catalytic converter. Furthermore, the testing of the catalytic converter can be carried out only under given operating conditions of the engine, for instance during idling. A large number of detailed methods are known for the amplitude evaluation.
Of the evaluation methods mentioned above, frequency evaluation is more sensitive than amplitude evaluation. Frequency evaluation systems, however, are less popular than amplitude evaluation systems because the testing of a catalytic converter by means of the frequency method has a strong effect on the quietness of operation of the engine and the emission of pollution gas, which is undesired.